This invention relates to large-size abrasive boron nitride particles produced by a shock wave process.
Boron nitride is similar to carbon in many respects. Both materials have a soft hexagonal crystal form possessing lubricating qualities. Both can be converted under high pressure conditions to two extremely hard forms--a hexagonal crystal form having a wurtzite crystalline structure and a cubic form having a zinc blende crystalline structure. The soft form of boron nitride has a specific gravity of about 2.28 while both of the hard forms have a specific gravity of about 3.45. The soft form will sometimes be referred to hereinafter as "low-density boron nitride" and the hard forms as "high-density boron nitride". The high-density hexagonal form will sometimes be referred to as "wurtzite boron nitride" and the cubic high-density form will sometimes be referred to as "zincblende boron nitride".
The preparation of zincblende boron nitride was disclosed and claimed in Wentorf U.S. Pat. No. 2,947,617 which disclosed a catalytic process for making the material. Later, Bundy and Wentorf described and claimed wurtzite boron nitride in U.S. Pat. No. 3,212,851 which disclosed a direct transformation process for making the material.
It has been known that low-density boron nitride can be converted to high-density boron nitride by a shock wave process. Example 7 of British Pat. No. 1,281,002 describes the production of high-density boron nitride by means of a shock wave created by the detonation of an explosive charge.
Pyrolytic low-density boron nitride was described in Basche U.S. Pat. No. 3,152,006 which disclosed the preparation of such material by contacting a substrate with the commingled vapors of ammonia and boron trichloride on a graphite substrate at a temperature of about 1900.degree. C. Later, Moore U.S. Pat. No. 3,578,403 described the recrystallization of pyrolytic boron nitride to produce a highly crystalline transparent material. In this process the pyrolytic boron nitride was subjected to a temperature above 2250.degree. C. under a pressure of between 5000 psi and 15,000 psi applied in a direction perpendicular to the basal planes of the pyrolytic material.
The static processes of U.S. Pat. Nos. 2,947,617 and 3,212,851 enable the high-density boron nitride to be formed over a period of a number of seconds or even minutes. As a result the high-density crystals are relatively large. For example, column 11, line 39, of U.S. Pat. No. 2,947,617 reports crystals having an average diameter of 200 to 400 microns. Contrasted with this, shock wave processes effect the transformation of low-density boron nitride to high-density boron nitride in a period of about a microsecond. Particle sizes reported from shock wave processes have been 10 microns diameter or less. This small size has greatly limited the commercial use of shock wave techniques for producing high-density boron nitride.